1. ## Find Cos

ok so i guess i am a little foggy from the summer break. can someoen help me with this question?

if cos t = (4/5), then find cos (t + pi)

2. $\displaystyle \cos(t+\pi)=\cos{t}\cos{\pi}-\sin{t}\sin{\pi}$

Take it from there.

3. Originally Posted by zooanimal98
ok so i guess i am a little foggy from the summer break. can someoen help me with this question?

if cos t = (4/5), then find cos (t + pi)

Let's try to see what $\displaystyle \cos (t + \pi)$ simplifies to, in terms of sine and cosine...

$\displaystyle \cos (t + \pi)=\cos t\cos\pi-\sin t\sin\pi$ <-- Sum Identity

Note that $\displaystyle \cos\pi=-1\text{ and }\sin\pi=0$

Thus, our expression simplifies to $\displaystyle \cos(t+\pi)=-\cos t$

I'm sure you can take it from here...

I hope this helps

--Chris

EDIT: JaneBennet beat me...

4. ## is the answer -4/5?

i guess i am a little on the slow side. does that mean the answer is -4/5?
if not then well i guess i still need a better explantion

Originally Posted by Chris L T521
Let's try to see what $\displaystyle \cos (t + \pi)$ simplifies to, in terms of sine and cosine...

$\displaystyle \cos (t + \pi)=\cos t\cos\pi-\sin t\sin\pi$ <-- Sum Identity

Note that $\displaystyle \cos\pi=-1\text{ and }\sin\pi=0$

Thus, our expression simplifies to $\displaystyle \cos(t+\pi)=-\cos t$

I'm sure you can take it from here...

I hope this helps

--Chris

EDIT: JaneBennet beat me...

5. Originally Posted by zooanimal98
i guess i am a little on the slow side. does that mean the answer is -4/5?
if not then well i guess i still need a better explantion

--Chris